Model based paleoclimate interpretations of Holocene oxygen isotope records from the Pacific Northwest

Abstract:

Oxygen isotope measurements of authigenic carbonate from Cleland Lake (British Columbia), Paradise Lake (British Columbia), and Lime Lake (Washington) provide an ~9,000 year Holocene record of precipitation-evaporation balance variations in the Pacific Northwest. Both Cleland Lake and Paradise Lake are small, surficially closed-basin systems with no active inflows or outflows. Lime Lake is surficially open with a seasonally active overflow. We sampled the lake sediment cores at 1-60 mm intervals (~3-33 years per sample on average) and measured the isotopic composition of fine-grained, authigenic CaCO₃ in each sample. Negative δ¹⁸O values, which indicate wetter conditions in closed-basin lakes, occur in Cleland Lake and Paradise Lake sediment during the mid-Holocene and are followed by more positive δ¹⁸O values, which suggest drier conditions, in the late Holocene. The δ¹⁸O record from Lime Lake, which principally reflects changes in the isotopic composition of precipitation, exhibits less variability than the closed-basin lake records and follows an increasing trend from the mid-Holocene to present. Power spectrum analysis of the Cleland Lake δ¹⁸O data from 1,000 yr BP to present demonstrates significant periodicities of ~6 and ~67 years that likely reflect the enhancement of El Niño Southern Oscillation (ENSO) variability in the late Holocene with an associated multidecadal (i.e., 50 to 70 yr) component of the Pacific Decadal Oscillation. Results from mid-Holocene (6,000 yr BP) climate model simulations conducted as part of the Paleoclimate Modeling Intercomparison Project Phase 3 (PMIP3) indicate that in much of western North America, the cold season was wetter, and the warm season (April–September) was considerably drier (relative to the late Holocene), leading to an overall drier climate in western North America but with enhanced hydroclimatic seasonality. This is consistent with inferences from the Cleland and Paradise Lake δ¹⁸O records, which lake modeling experiments indicate are strongly influenced by cold season precipitation-evaporation balance. This also helps to explain apparent inconsistencies between the lake δ¹⁸O records and other proxies of hydroclimatic change from the greater Pacific Northwest region that indicate relatively drier conditions during the mid-Holocene.